How to Capture a Data Packet
Trace with Wireshark
Reference: AN2138
Date: 2 May 2012
Product Type: Switched Ethernet Networks
Purpose
This application note will help you capture data packets on a switched Ethernet network.
Introduction
The complex nature of a communications network can make troubleshooting problems somewhat elusive. One
useful procedure to find an elusive application problem is to capture and analyze the actual raw data that is
traversing a network. This captured data is called a “data packet” or “packet trace.” Also, software developers
occasionally request a packet trace to analyze application performance/operation of equipment on customer
premises.
Generally, you capture packet traces with a laptop (as we show in the examples). Because of the mobility of a
laptop, you can capture a packet trace from any point on a network—although most desktop personal computers
can also do this. A packet capture application (usually called a “sniffer”) is also necessary for this procedure; in
this application note we use Wireshark (http://www.wireshark.org/).
Note: We will use several applications in this note but will not give the installation procedures. These
applications have standard installation procedures and our use here will not deviate from those norms.
Before we give procedures to attach and configure the laptop and software, there is a problem that is necessary
for you to know—and to know how to solve—as described in the subsequent two sections:
Problem: The Network will not Naturally Forward Certain Packets to
the Laptop
Because Ethernet switches only forward
packets to the destination address
contained in the packet header, there is a
problem when you are plugged into one
port of a switch while attempting to
capture:
 Packets that are destined for a machine
plugged into a different port or
 The data packets on a backbone port—
for example, a fiber port as shown here.
If the network is operating correctly, the
laptop cannot capture all data packets that
are destined for the signal controller
(ASC/3) or traversing the network backbone link (fiber). The laptop can only detect broadcast packets, unicast
packets destined for the laptop itself and, in some network configurations, multicast traffic.
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AN2138: How to Capture a Data Packet Trace with Wireshark
Two Possible Solutions to the Problem
1. Connect to a Hub – If it is necessary
to capture packets destined for the signal
controller, you can temporarily install a
hub as shown here—but you cannot
capture packets on backbone links with
this solution.
A hub is a network repeater that
forwards all packets out of all ports at all
times. Thus the laptop would detect and
be able to capture packets destined to go
to or from the signal controller.
Hubs are inefficient and can be difficult
to find.
2. Enable Port Mirroring – You can enable your Ethernet switch to mirror (copy) data packets on one switch
port to a second switch port. Different manufacturers refer to this function in different terms. RuggedCom refers
to this as Port Mirroring and we use its switch in our example. Cisco refers to this as Switched Port Analyzer
(SPAN) port and other manufacturers refer to this as Roving Analysis.
Notes
Necessary information before you start the procedure that follows on the subsequent pages:

For this procedure, your laptop must have a serial port and most laptops do not have a serial port. The
easiest way to configure your laptop with a serial port is to buy and install a USB to DB-9 (serial port)
adapter. The steps to do this depend on the manufacturer of that device and are outside of the scope of
this application note.

If you use an Ethernet switch manufacturer other than RuggedCom, its procedure will be different.

If you use a terminal application other than Tera Term, its procedure will be different.
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AN2138: How to Capture a Data Packet Trace with Wireshark
Procedure to Enable Port Mirroring
1. As shown here, connect an
RS-232 serial cable from the
laptop serial port to the console
port of the switch.
2. Obtain a terminal application to
enable the port monitoring
feature in the switch. In our
example, we use Tera Term.
Currently, this is free opensource software that you can
download from:
http://en.sourceforge.jp/projects/ttssh2/
3. Launch the Tera Term terminal
application.
4. You should see the configuration window shown here.
5. As shown, select the serial port to which you are
connected.
6. Click OK .
Note: Switch settings are usually silkscreened on the
RuggedCom units as shown below.
7. From the setup menu, select “Serial Port.”
8. You should see the port setup window shown here.
9. Configure your serial port to match the settings of the
switch.
10. Click OK .
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AN2138: How to Capture a Data Packet Trace with Wireshark
11. Press Enter .
12. The login screen comes into view.
13. Enter you user name and password.
Factory default is:
User name: admin
Password: admin
14. From the “Main Menu” select
Ethernet Ports > Configure Port
Mirroring.
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AN2138: How to Capture a Data Packet Trace with Wireshark
Now we will enable port mirroring and
indicate:
 Where the laptop/sniffer is
connected (that will be our “Target
Port”) and
 The device to capture data to/from—
in our example an ASC/3.
15. Refer back to the illustration on Page 1.
16. Note that the ASC/3 is connected to
Port 6 and the laptop is connected to
Port 2.
17. For our example, the configuration will
look like this (although your setup is
probably different).
If you try to capture data packets from a
backbone port, then the source ports will
be that port. The fiber port in our
example would be Port 7.
Note: There are two entries for our source port labeled “Egr” and “Ingr”:
Egr = Egress (departure point)
Ingr = Ingress (entrance point)
Usually, your interest will be one physical Ethernet port at a time and the number of these entries will be
the same. Occasionally, you might desire to trace entry packets from one interface and departure packets
from another interface. You can also configure this for these rare applications.
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AN2138: How to Capture a Data Packet Trace with Wireshark
Taking a Trace
Wireshark is a very powerful tool that has many different configuration parameters for a number of different
applications. We will focus on what we consider to be the three most common uses:
 Standard Packet Trace
 Filtered Packet Trace
 Snapshots
Standard Packet Trace – Use this to take a quick look at network traffic for purposes of general discovery
and/or verification.
Filtered Packet Trace – You filter out (remove from view) all packets except those you want to see.
Snapshots – Packet traces saved to disk on an interval, usually a set amount of time.
Standard Packet Trace
We will now start the sniffer
application. If you are using Wireshark,
you will see a screen similar to this.
Note several things from this initial
screen (the areas circled in red):
Interface List – This list should show
the Ethernet port on your laptop with
which you will capture data packets.
How to Capture – Make sure you have
internet access before you select this.
This link will take you to a website with
much more detailed information on how
to set up Wireshark and packet capture
options.
User’s Guide
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AN2138: How to Capture a Data Packet Trace with Wireshark
1. From the “Interface List,” select your Network Interface Card.
Note: For help to know which interface listed relates to your physical Ethernet port, refer to the
Appendix at the end of this application note.
2. Now you will see packets streaming
down a screen similar to what you
see on this screen.
This will capture any and all data
packets entering or departing the
particular Ethernet port to which
you are attached. The amount of
data packet traffic you see will
probably be overwhelming. Thus,
you might wish to narrow down
your packet trace to a set of
criteria—for example, one IP
address.
The next section tells you how to filter out all packets except those you want to see.
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AN2138: How to Capture a Data Packet Trace with Wireshark
Filtering with Wireshark
In most cases, you will want a filtered
packet trace that shows packets from
one particular device. For example, you
can filter by IP address and
capture/display only packets that are
destined for or originate from that
particular address.
1. As shown at the top of this screen,
from the “Capture” menu select
“Capture Filters.”
2. You will see a screen similar to this.
3. Enter the “Filter name” in the box at the
bottom of the screen.
Notice that there are many examples of
predefined filters from which you can select.
But the “Filter name” is a variable. That is,
you can name a filter anything you like.
4. Enter the “Filter string” in the next box. As
you would imagine, the “Filter string”
values must follow a very strict yet simple
set of rules. For one host IP address (as in
the example here), you enter:
host
one space
the IP address
Make sure there are no extra characters such
as a space after the IP address.
5. When you are satisfied with your filter
string, click OK .
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AN2138: How to Capture a Data Packet Trace with Wireshark
6. After you are satisfied that you have
defined your capture filter correctly,
you can start your packet trace.
Notice that the packets shown are
destined for or originate from the IP
address defined in our capture filter.
In this example, you can see data
flowing between a Centracs server and
an ASC/3.
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AN2138: How to Capture a Data Packet Trace with Wireshark
Taking Snapshots
Often, network issues are time/event
based and/or triggered. That is, issues
that are happening infrequently but are
not necessarily predictable can be
difficult to catch. It can be advantageous
in these instances to take periodic
snapshots of packet data. When the
event occurs, you can inspect the related
packet trace.
We will demonstrate here how you
would configure Wireshark to capture
packet traces in 5-minute intervals.
1. As shown at the top of this screen,
from the “Capture” menu select
“Options.”
2. You should see the screen shown here.
In the three highlighted sections of this
screen (the areas circled in red)—from top
to bottom:
 Make sure you select the correct
Ethernet interface; refer to the
Appendix.
 You can define a capture filter from this
screen if you want (we will not use one
in this example).
 In “Capture File(s)” we will configure
Wireshark to take snapshots.
3. To select the directory into which Wireshark
will save snapshot files, click Browse…..
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Before you continue to the details of this process, you should create an empty folder in which to store your
sniffer trace files. Anywhere that makes sense to you is fine. We recommend you keep all trace files in a
common location, for example:
C:\Sniffer\CityName\
4. Browse to the directory you created
previously and enter a file name in
the available field titled “Name”.
Make sure you enter the extension
pcap on the file name as shown here.
Notice Create Folder on the right
side of the screen. You can also use
this button to create your directory.
5. Select OK and return to the
“Capture Options” screen.
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6. Notice that the path and file name in
the file entry window match our last
selection.
7. To complete the configuration of this
screen and start the process, click to
check these boxes and enter numbers
as shown:
√ Use multiple files
√ Next file every—5 minute(s)
√ Ring buffer with 72 files
Use multiple files – This is rather selfexplanatory; it enables us to save more
than one packet capture file.
Next file every—minute(s) – Our goal is
to have snapshots of network traffic every
5 minutes, which is defined here.
Ring buffer with 72 files – This defines
the maximum number of files that will be
kept.
In our example, we can keep a maximum
of 6 hours of snapshots. These calculations
show you how 72 files store 6 hours:
1 file every 5 minutes X 72 = 360 minutes
= 6 hours
Verify Function
To verify function:
1. Allow some time to pass.
2. Inspect the directory you created. It
should show files stacking up as shown
here.
Note: The file names start and end with the
naming convention you defined. The
portion of the file name that Wireshark
inserted is an index number and date/time
stamp.
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Appendix
The purpose of this appendix is to help
you know which device to select from
the “Interface list.”
Even though your laptop might only
have one Ethernet port, it is likely to
have a number of devices listed here.
The easiest way to know which one of
these entries represents your physical
Ethernet port is to check your “Network
Connections” screen shown here.
In most cases it will be the connection
labeled “Local Area Connection”. Also,
it will not be grayed out as most of the
other interfaces. Notice that the model
number of this interface matches the
initial entry from the “Interface List” in
the initial Wireshark screen. This is your
physical Ethernet port.
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Another procedure to know which one
of these entries represents your physical
Ethernet port:
1. Select “Interfaces” from the
“Capture” menu.
2. A menu similar to this comes up.
Information given in this menu:

Your IP addresses are listed here
which might clarify things for
you.

You will see the packet counters
increasing, which should
indicate which Ethernet
interface is plugged in and
receiving data.
Note: From this screen, to start a packet trace, click the Start of its Interface.
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